Supervisor:
Frede Blaabjerg

Co-Supervisor:
Xiongfei Wang

Assessment Committee:
Baoze Wei(Chair)
Professor Akshay Rathore, Singapore Institute of Technology
Professor Soledad Bernal-Perez, Universitat Politecnica de Valéncia, Spain

Moderator:
Baoze Wei

Abstract:

The evolution of societal needs in the 21st century across economic, social, and technological dimensions has been in parallel with an increased energy demand. This phenomenon necessitates the transformation of the existing power grid in order to follow a sustainable path. Therefore, wind power technology has seen significant employment in recent years, and it now constitutes a primary renewable energy source. This shift marks a transition from traditional fossil fuel-based energy generation, leading to rapid advancements in wind turbine technology and an increased share of wind power in global energy production. Power electronic converters play a crucial role in integrating wind farms into the main grid by aligning the demands of the grid with those of wind turbines.

Nevertheless, the transition from synchronous machine-based power systems to converter-based power systems has introduced new stability challenges. These challenges, potentially affecting the system’s normal operation, include new categories of system instabilities associated with wind turbine technology. Small-signal dynamics have been identified as a significant cause of instabilities in both low and high-frequency ranges. The focus of this PhD thesis is on the small-signal modeling of wind turbine and wind farm systems, and includes the design and control structure of wind turbine models which are used for implementing stability analysis studies.

Specifically, this PhD thesis investigates the grid-following converter control on the grid-side of wind turbines, with state-space modeling employed for designing the system’s small-signal model. The stability of the system is evaluated under various grid strengths, where the tuning of the controllers plays a critical role in ensuring stable operation. In addition, the stability impact of the alternating voltage controller’s (AVC) design on the system’s stability is under study. The bandwidth of the controllers is examined, both in the inner and outer loops of grid-following converters in wind turbines, in order to ensure the stability boundaries in each test case. This approach is first utilized to analyze the stability in a single grid-side converter of a wind turbine, and then is expanded to systems with multiple converters of wind turbines. All the examined test cases regarding the stability analysis of the wind turbine’s small-signal model are validated by corresponding time domain simulations, ensuring the conclusions of this study.

In addition to controller tuning, the PhD thesis explores hardware solutions in stability analysis. Notably, it addresses the emergence of subsynchronous oscillations observed in large wind farms in recent years. For that purpose, an aggregated wind farm model is used, and the stability impact of incorporating a synchronous condenser into the wind farm’s model is examined, particularly in weak grid conditions, to mitigate existing subsynchronous oscillations. Here, the influence of the synchronous condenser’s power rating on system stability is highlighted, demonstrating that an inappropriate selection may not adequately mitigate subsynchronous oscillations. The significance of the wind farm’s small-signal modeling is underscored, in which a synchronous condenser state-space model is included, as the optimal range of the synchronous condenser’s rating is determined for the wind farm system under study.

The findings of this thesis are anticipated to be valuable for future research on the stability of wind farms connected to the main grid, as well as for the industrial aspects of wind power technology